Close encounters with Japan's 'living fossil'
"This is a dinosaur, this is amazing," he enthuses.
"We're talking about salamanders that usually fit in the palm of your hand. This one will chop your hand off."
As a leader of Conservation International's (CI) scientific programmes, and co-chair of the Amphibian Specialist Group with the International Union for the Conservation of Nature (IUCN), Dr Gascon has seen a fair few frogs and salamanders in his life; but little, he says, to compare with this.
Fortunately for all of our digits, this particular giant salamander is in no position to chop off anything, trapped in a tank in the visitors' centre in Maniwa City, about 800km west of Tokyo.
But impressive it certainly is: about 1.7m (5ft 6in) long, covered in a leathery skin that speaks of many decades passed, with a massive gnarled head covered in tubercles whose presumed sensitivity to motion probably helped it catch fish by the thousand over its lifetime.
If local legend is to be believed, though, this specimen is a mere tadpole compared with the biggest ever seen around Maniwa.
A 17th Century tale, related to us by cultural heritage officer Takashi Sakata, tells of a salamander (or hanzaki, in local parlance) 10m long that marauded its way across the countryside chomping cows and horses in its tracks.
A local hero was found, one Mitsui Hikoshiro, who allowed the hanzaki to swallow him whole along with his trusty sword - which implement he then used, in the best heroic tradition, to rend the beast from stem to stern.
It proved not to be such a good move, however.
Crops failed, people started dying in mysterious ways - including Mr Hikoshiro himself.
Pretty soon the villagers drew the obvious conclusion that the salamander's spirit was wreaking revenge from beyond the grave, and must be placated. That is why Maniwa City boasts a shrine to the hanzaki.
The story illustrates the cultural importance that this remarkable creature has in some parts of Japan.
Its scientific importance, meanwhile, lies in two main areas: its "living fossil" identity, and its apparently peaceful co-existence with the chytrid fungus that has devastated so many other amphibian species from Australia to the Andes.
"The skeleton of this species is almost identical to that of the fossil from 30 million years ago," recounts Takeyoshi Tochimoto, director of the Hanzaki Institute near Hyogo.
"Therefore it's called the 'living fossil'."
The hanzaki (Andrias japonicus) only has two close living relatives: the Chinese giant salamander (A. davidianus) , which is close enough in size and shape and habits that the two can easily cross-breed, and the much smaller hellbender (Cryptobranchus alleganiensis) of the south-eastern US.
Creatures rather like these were certainly around when dinosaurs dominated life on land, and fossils of the family have been found much further afield than their current tight distribution - in northern Europe, certainly, where scientists presumed the the lineages had gone extinct until tales of the strange Oriental forms made their way back to the scientific burghers of Vienna and Leiden a couple of centuries ago.
"They are thought to be extremely primitive species, partly due to the fact that they are the only salamanders that have external fertilisation," says Don Church, a salamander specialist with CI.
The fertilisation ritual must be quite some sight.
Into a riverbank den that is usually occupied by the dominant male (the "den-master") swim several females, and also a few other males.
The den-master and the females release everything they have got, turning incessantly to stir the eggs and spermatozoa round in a roiling mass.
Maybe the lesser males sneak in a package or two as well; their function in the ménage-a-many is not completely clear.
When the waters still, everyone but the den-master leaves; and he alone guards the nest and its juvenile brood.
It is not an ideal method of reproduction.
Research shows that genetic diversity among the hanzaki is smaller than it might be, partly as a result of the repeated polygamy, which in turn leaves them more prone to damage through environmental change.
But for the moment, it seems to work.
Outside the breeding season, the salamander's life appears to consist of remaining as inconspicuous as possible in the river (whether hiding in leaves, as the small ones do, or under the riverbanks like their larger fellows) and snapping whatever comes within reach, their usual meandering torpor transformed in an instant as the smell of a fish brushes by.
The adults' jaws are not to be treated lightly.
Among Dr Tochimoto's extensive collection of photos is one of bloodied human hands; and as he warns: "you may be attacked and injured; please be careful".
When the chytrid fungus was identified just over a decade ago, indications were that Japan would be an unlikely place to look for its origins.
With the discovery of chytrid on museum specimens of the African clawed frog (Xenopus laevis) , an out-of-Africa migration spurred by human transportation of amphibians once seemed the simple likelihood.
But just last year, a team of researchers led by Koichi Goka from Japan's National Institute for Environmental Studies published research showing that certain strains of chytrid were present on Japanese giant salamanders, and only on Japanese giant salamanders, including museum specimens from a century or so back; and that the relationship seemed benign.
The hanzaki-loving strains of chytrid appear to differ from those that are proving so virulent to amphibians now.
Unravelling all that, says Don Church, might tell us something about the origins and spread of chytrid - and there is so much diversity among Japanese chytrid strains that the country is now being touted as a possible origin, as diversity often implies a long evolutionary timeframe.
More importantly, the discovery might also provide options for treating the infection.
"In the case of the North American salamanders, what was found was that they have bacteria living on their skin that produce peptides that are lethal to the amphibian chytrid fungus," says Dr Church.
"And those bacteria might be able to be transplanted to other species that can't fight off the fungus."
This is a line of research that is very much in play in laboratories around the world.
It appears likely now that studies of the Japanese giant salamander can expand the number of chytrid-fighting bacteria known to science, and so extend the options for developing treatments for an infection that currently cannot be controlled in the wild.
But that can only come to pass if the giant salamanders endure; something that is not guaranteed, with the challenges they face in modern Japan including, perhaps, new strains of chytrid itself.
There is as yet no modern hero able to still the pace of habitat loss or prevent invasion from rival species.
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